In apparatuses of the prior art which mount electronic components on printed boards and other wiring boards, various methods are employed to bond the electrodes of the electronic components to the electrodes of the wiring boards; one known method of mounting electronic components in a short time and at comparatively low temperatures is a bonding method employing ultrasound (hereafter called ultrasonic bonding).
In ultrasonic bonding, an electronic component pressed against a wiring board is vibrated by ultrasonic vibration, and the electrodes of the electronic component (with bumps formed, for example) and the electrodes of the wiring board are electrically bonded. At this time, the bumps may be formed on the electrodes of the wiring board, or may be formed both on the electrodes of the electronic component and on the electrodes of the wiring board.
Explanatory diagrams of (a) and (b) in FIG. 13 show a state in which conventional ultrasound is used to perform bonding of the metal electrodes of an electronic component 4 to the metal electrodes of a wiring board 14 using an electronic component mounting apparatus (hereafter called an “ultrasonic bonding apparatus”), which mounts electronic components 4 onto wiring boards 14; 1 is an ultrasonic vibrator, 2 is an ultrasonic horn, and 3 is a tool.
In (a) of FIG. 13, a cross-sectional view is shown, and in (b) of FIG. 13 the figure on the right is a base view with the wiring board 14 omitted, while in (b) of FIGS. 3, 9 is the figure on the left is an ultrasound amplitude with no loading, and the quantity of transmission of ultrasonic vibration during the application of a load is shown in a pseudo manner.
As shown in (a) of FIG. 13, ultrasonic vibration generated by the ultrasonic vibrator 1 is transmitted to the electronic component 4 via the ultrasonic horn 2 that transmits the ultrasonic vibration and via the tool 3 in direct contact with the loaded electronic component 4, and the electrodes on the electronic component 4 are bonded with the electrodes of the wiring board 14.
FIG. 14 is an explanatory diagram showing the relation between loading conditions for loading the bonded portions of the electronic component and ultrasonic vibration conditions, and non-defective and defective items of the bonded portions of the electronic component. The horizontal axis represents the loading conditions for loading each bonded portion, and the vertical axis represents the ultrasonic vibration.
As shown in FIG. 14, in an area B of light loading or of low ultrasonic vibration, the load energy is insufficient for bonding, so that there is electrical discontinuity at bonded portions where the electrodes of the electronic component 4 and the electrodes of the wiring board 14 are not bonded, or there is insufficient bonding strength to secure the reliability required of a product (for example, the shear strength of the electronic component 4 with respect to the wiring board 14), or other bonding defects arising from insufficient bonding strength occur.
Further, in an area C of high loading or of high ultrasonic vibration, excessive loading energy causes breakage of the electronic component 4 or of the wiring board 14.
Hence there is a need to set the load value and ultrasonic vibration value, as the bonding conditions, within a range in which the bonding strength needed to secure the reliability demanded of products can be secured, and moreover breakage of the electronic component 4 or of the wiring board 14 does not occur. In an area A it is possible to achieve favorable bonding of the electronic component 4 and wiring board 14 with an appropriate load and appropriate ultrasonic vibration.
Patent Reference 1: Japanese Patent Application Laid-Open No. 2004-330228
In recent years, ultrasonic bonding methods have been recognized as superior, in enabling bonding of electronic components in short times and at comparatively low temperatures, over other bonding methods for bonding the electrodes of electronic components and the electrodes of wiring boards, such as bonding methods in which bumps formed on the electrodes of electronic components are bonded to bumps formed on the electrodes of wiring boards via conductive adhesive, bonding methods in which bumps formed on the electrodes of electronic components and the electrodes of wiring boards are bonded via adhesive sheet comprising conductive particles, or bonding methods in which solder bumps formed on electronic components are bonded to the electrodes of wiring boards; and so the application of ultrasonic bonding to numerous forms of electronic component bonding is anticipated.
In particular, in forms of electronic component bonding of driver ICs for image displays and other large components, in which the electrode pitch is expected to continue to decrease, thermal expansion of constituent members due to thermal loading at the time of bonding causes shifts in bonding position and other problems, so that great expectations are being placed on ultrasonic bonding methods enabling bonding at low temperatures.
In ultrasonic bonding, in order to transmit ultrasonic vibrations from the tool to the entire face of the electronic component, the lengths of the tool perpendicular to and parallel to the ultrasonic vibration direction are set to be the same as the lengths of the electronic component perpendicular to and parallel to the ultrasonic vibration direction. Alternatively, the lengths of the tool perpendicular to and parallel to the ultrasonic vibration direction of the electronic component are set to be longer than the lengths perpendicular to and parallel to the ultrasonic vibration direction. Moreover, the lengths of the tool perpendicular to the ultrasonic vibration direction are set to be the same at the component-holding face of the tool, and the lengths of the tool parallel to the ultrasonic vibration direction are set to be the same at the component-holding face.
However, given such a tool shape, the farther a region in the tool from the center line of the tool parallel to the ultrasonic vibration direction, the poorer the transmission of ultrasonic vibrations.
That is, as shown in (b) of FIG. 13, changes occur in the ultrasound amplitude 9 at the center portion of the tool 3 and the ends of the tool 3, that is, a difference occurs in transmission of ultrasonic vibration. In particular, at the ends of the tool 3, the transmission of ultrasonic vibrations is extremely small.
As explained above, the bonding conditions of ultrasonic bonding has to be bonding conditions in the area A, in which the electrodes of the electronic component are bonded to the electrodes of the wiring board, and no breakage occurs; but if a difference occurs in the transmission of ultrasonic vibrations to the electrodes in the center portion of the electronic component and to electrode portions at the ends, the bonding conditions to be actually imposed at different bonded portions in the electronic component have a range, for example, as shown in FIG. 14, and it is not possible to find bonding conditions satisfying both the requirement of securing bonding strength at all the bonded portions in the electronic component and the requirement that there be no breakage of the electronic component or the wiring board. In FIG. 14, a range D1 represents a range of actually imposed bonding conditions at ends perpendicular to the ultrasonic vibration direction of the electronic component. A range D2 represents a range of actually imposed bonding conditions at areas other than the ends perpendicular to the ultrasonic vibration direction of the electronic component.
Alternatively, even if bonding conditions are discovered satisfying both the requirement of securing bonding strength at all bonded portions in the electronic component, and the requirement that there be no breakage of the electronic component or of the wiring board, the bonding conditions have small margins for securing non-defective products, and the maintenance of the bonding conditions in mass production is extremely difficult.